The most important step in building your online business is without a doubt: market research.
Find the right niche and the rest is easy. Find a niche with too much competition and or where people just are not interested in buying anything online and you're business will sink faster than a bowling ball thrown overboard.
Market research is where the real decision is made as to who will prosper and whose business will wither away.
Even though it's that important, people ignore this vital step because it seems boring.